The use of lithium ion secondary batteries as power supplies for portable electronic equipment is rapidly spreading. Carbon powder is used as the negative electrode material of a nonaqueous secondary battery typified by a lithium ion secondary battery. Due to the demand for small batteries with higher capacity, it has been attempted to increase the discharge capacity of negative electrode materials. To this end, synthetic graphite powder having a high degree of graphitization is primarily used as a negative electrode material for the current nonaqueous secondary batteries. In addition, in order to further increase the discharge capacity of a battery per unit volume, it has been attempted to increase the packing density of electrodes so as to make the electrode density higher.
There is another desire to lower the cost of negative electrode materials for nonaqueous secondary batteries. Particularly in large-sized batteries such as those for automobiles, since a large amount of negative electrode materials is used, there is an increasing demand for lower their costs. On this account, attempts have been made to use natural graphite, which is inexpensive and which has a high degree of graphitization and a high true specific gravity, in place of expensive synthetic graphite.
However, due to its extremely high degree of graphitization, natural graphite has problems such as a high reactivity with an electrolytic solution, which leads to an increase in irreversible capacity caused by decomposition of the electrolytic solution, and deterioration of battery properties such as shelf stability and safety. Recently, in batteries for use in automobiles such as electric cars or hybrid cars, since they sometimes must operate at low temperatures, it has been attempted to use an electrolytic solution containing propylene carbonate (abbreviated below as PC), which is a low melting point liquid (melting point −49° C.), as a nonaqueous solvent. However, graphite powder having a high degree of graphitization causes decomposition of PC, so natural graphite powder cannot be used as it is in batteries as a negative electrode material in combination with a PC-based electrolytic solution (i.e., a nonaqueous electrolytic solution containing propylene carbonate as a solvent).
In order to suppress the reactivity of graphite powder having a high degree of graphitization with an electrolytic solution, there have been many attempts to use a multilayered carbon powder prepared by coating the surface of graphite powder with a carbonaceous material having a low degree of graphitization called turbostratic carbon or low temperature calcined carbon.
JP H08-50897 A1 discloses carbon powder prepared by heating a carbon precursor such as pitch to melt, mixing the resulting melt with graphite powder, and subjecting the mixture to heat treatment at a low temperature to form carbon powder in which the surface of the graphite powder is coated with turbostratic carbon having a low degree of graphitization (low temperature calcined carbon).
JP H04-368778 A1 discloses carbon powder formed by depositing turbostratic carbon with a low degree of graphitization on the surface of graphite powder by the chemical vapor deposition (CVD) method.
Each of the above-described multilayered carbon powders is based on the concept of using graphite powder as a core and coating its entire surface with carbon having a low degree of graphitization in order to suppress its reactivity with an electrolytic solution. Therefore, a large amount of carbon having a low degree of graphitization is used as a coating material.
Carbon having a low degree of graphitization such as low temperature calcined carbon exhibits a gradual change in electrode potential. In addition, it has charge and discharge voltages vs lithium which are higher than those of graphite. Therefore, the use of a multilayered carbon powder having a core of graphite coated with such carbon result in a decreased battery voltage compared to current batteries using graphite alone as a negative electrode material. Accordingly, under actual conditions of use, the multilayered carbon powder produces a battery with a decreased discharge capacity and decreased charge-discharge efficiency. In addition, carbon having a low degree of graphitization such as low temperature calcined carbon has a low true specific gravity compared to graphite, and it is extremely hard. Therefore, with the multilayered carbon powder, the density of an electrode cannot be sufficiently increased by compression, and the discharge capacity per unit volume becomes smaller than that of graphite powder. Furthermore, when pitch which melts when heated is used for coating, if a large amount of pitch is used, the amount of liquid phase formed by heating so increases that aggregation of powder particles occurs during heat treatment. As a result, there is the problem that an additional grinding step becomes necessary, resulting in an increase in costs.
For this reason, it has been proposed to coat a core of graphite powder with a limited amount of carbon having a low degree of graphitization such as low temperature calcined carbon.
JP 2000-58052 A1 discloses a method of manufacturing a carbon material in which graphite powder is immersed in a melt of carbon precursor such as pitch, and after washing the powder with a solvent to remove excess carbon precursor deposited thereon, it is heated to carbonize the deposited carbon precursor. JP H09-213328 A1 discloses a method in which graphite powder is mixed with a carbon precursor such as pitch in a solvent, and the mixture is heated with stirring to remove the solvent and then calcined for carbonization.
These two methods are also based on the concept of using graphite powder as a core material and coating the entire surface of the graphite powder with a carbon powder having a low degree of graphitization to suppress a reaction with an electrolytic solution. In these methods, pitch, which is a carbon precursor, is used in a liquid phase for contact with graphite powder. Therefore, a portion of the pitch is consumed for filling the relatively large pores of the graphite powder. As a result, pitch has to be use in a considerably large amount. If the amount of pitch which is used is insufficient to completely coat the surface of the graphite powder, a portion of the surface of the graphite powder is exposed, and the charge-discharge characteristics in PC-based electrolytic solution cannot be improved to a desired level.
A common problem in the technology disclosed in the above-described patent documents is that due to coating of the entire surface of graphite powder with carbon having a low degree of graphitization, the contact resistance between particles of the resulting carbon powder increases, leading to a decrease in rate capability (i.e., rapid charge characteristics and high rate discharge characteristics) of batteries.
JP 2003-100292 A1 proposes a method in which pitch powder and graphite powder are simply mixed in a solid phase and the mixture is then subjected to heat treatment at a temperature of 600 to 800° C. Since that method is intended to obtain a negative electrode material having a capacity exceeding the theoretical capacity of graphite (372 mAh/g), pitch powder is used in a large amount for mixing with graphite powder. The electrochemical properties of the negative electrode material were tested using a PC-free electrolytic solution, so its charge-discharge characteristics in PC-based electrolytic solutions is unknown. Because this material contains a large amount of pitch-derived low temperature calcined carbon having a low degree of graphitization, it unavoidably has the problems that the battery voltage cannot be increased and that the discharge capacity per unit volume cannot be improved due to its electrode density which cannot be increased.
JP 2003-272625 proposes a graphite material having a controlled pore volume. Specifically, it proposes a graphite material in which the ratio V2/V1 is in the range of 2.2-3.0, wherein V1 is the pore volume of pores having a diameter of 4-10 nm and V2 is the pore volume of pores having a diameter of 30-100 nm. That patent document explains that charging load characteristics are deteriorated when V2/V1 is above this range, while if it is below this range, the specific surface area of the material becomes so large that the initial efficiency of a battery decreases. The pore volume in that patent document is determined from the pore size distribution on the adsorption side measured by the BJH (Barrett-Joyner-Halenda) method using nitrogen adsorption. This graphite material is prepared by surface oxidation treatment of graphite particles at 500-1500° C. There is no description in that document concerning electrode density or charge-discharge characteristics in PC-based electrolytic solutions.